(1) Field of the Invention
The present invention relates generally to controlling transmitters such as sonar transmitters, and more particularly to a method and system for controlling the output of an array of transmitters in order to achieve a desired beam pattern.
(2) Description of the Prior Art
In sonar applications using an array of transmitters, it is desirable for each transmitter to be transmitting the same power at any given time in order to produce a beam pattern that maximizes the transmitted energy for a given power constraint. To provide a background to beamforming, the following briefly describes a typical situation that requires beamforming and a number of approaches that can be used.
Consider a plurality of transducers where each transducer transmits a signal that can be adjusted in amplitude and delayed in time. Together, the transmitted signals add constructively in certain directions and destructively in other directions. This variation in the gain profile provides a beam pattern. A typical beam pattern has a main lobe in the main axis and a number of side lobes in the off-axis directions. It is desirable in some applications to change the width of the main lobe and/or reduce the level of the side lobes. These types of adjustments are known as beam shaping.
One approach to beam shaping is to change the spacing between the transmitters. However, this approach involves a mechanical change to achieve a new beam pattern and is therefore not suitable for adaptive situations. Alternatively, one can electronically alter the beam pattern by changing the amplitude (i.e., amplitude shading) or the time delays (e.g., phase shading for a narrowband signal) associated with the transmitted signals.
In amplitude shading, the amplitudes of the transmitted signals are weighted to achieve a certain beam shape. For example, the amplitude can be tapered to achieve a wider main lobe and lower side lobe levels using well-known weighting functions such as the Binomial, Taylor or Dolph-Chebyshev weighting functions. However, this type of amplitude shading is generally only effective for a uniformly-spaced linear array of transmitters, and is therefore not appropriate for use with non-uniform or arbitrary array of transmitters. Also, extensions of these approaches to a two-dimensional array are not a direct process. Moreover, the tapering of the amplitudes usually causes a loss in the transmitted energy as attenuations are applied to the transmitters.
To maintain a high transmit level and to form beams at the same time, time-delay beamforming or phase shading for a narrowband signal can be used where the relative delays of the transmitted signals are used to form beams. This approach, however, lacks a direct analytic solution that relates the desired beam pattern to the time delays.